This Day in Mexican History: Aztec Calendar Unearthed 225 Years Ago 225 years ago today, on December 17, 1790, the Aztec Calendar Stone was unearthed in Mexico City’s Zócalo. Workers were laying water pipes in the south side of the plaza when they found the 24 ton colossal stone carving. Soon after, it was mounted on the western wall of the Metropolitan Cathedral where it remained for almost a hundred years until 1885 when officials transferred it to the Monolith Gallery of the National Museum i